1. Field of Invention
The present invention relates to angle of incidence filtering. In particular, the present invention relates to limiting the angle of incidence of light incident towards a dichroic band pass filter in connection with an implantable optical sensor.
2. Discussion of the Background
A sensor may be implanted within a living animal (e.g., a human) to measure the concentration of an analyte (e.g., glucose or oxygen) in a medium (e.g., interstitial fluid (ISF) or blood) within the living animal. Examples of implantable sensors employing indicator molecules to measure the concentration of an analyte are described in U.S. Pat. No. 6,330,464, which is incorporated herein by reference in its entirety.
FIG. 1 illustrates a cross-sectional view of an example of an existing sensor 100. FIG. 2 illustrates a cross-sectional view of the existing sensor 100 in operation. The sensor 100 includes a light source 108 that emits excitation light 129 to a graft 106 containing indicator molecules. The indicator molecules have an optical characteristic that varies based on the concentration of the analyte in the medium. In particular, when excited by the excitation light 129, indicator molecules that have bound the analyte emit (i.e., fluoresce) light having a wavelength different than the wavelength of the excitation light 129. The sensor 100 includes a first dichroic band pass filter 111 (thin film) that filters light incident on a first photodetector 110. The first dichroic band pass filter 111 is designed to only pass light having the wavelength of the light emitted by the indicator molecules so that, in theory, the first photodectector 110, which is a signal photodetector, only receives the light emitted by the indicator molecules.
In sensors having multiple channels (e.g., a signal channel and a reference channel) and/or multiple photodiodes, the sensor may include a dichroic band pass filter for each channel and/or photodetector. For instance, as shown in FIGS. 1 and 2, existing sensor 100 includes a second dichroic band pass filter 113 (thin film) that filters light incident on a second photodetector 112. The second dichroic band pass filter 113 is designed to only pass light having the wavelength of reference light so that, in theory, the second photodetector 112, which is a reference photodetector, only receives the reference light.
In the existing sensor 100, the dichroic band pass filters 111 and 113 are coated directly onto the surface of each photodetector (e.g., photodiode). In existing sensor 100, light (e.g., reflected excitation light 129 and fluorescent light emitted by the indicator molecules in the graft 106) passes through one or more glass windows 220 and 222. Each of the glass windows 220 and 222 may be a clad rod, which may be glued onto a dichroic band pass filter coated photodetector using optical epoxy.
As noted above, excitation light 129 from the light source 108 serves to excite the fluorescent indicator molecules within the surface graft 106. When excitation light 129 is absorbed by the indicator molecules, its energy is converted to analyte modulated fluorescence, which may be detected by the signal channel (e.g., including first photodetector 110). For excitation light 129 that is not absorbed (e.g., because it did not encounter an indicator molecule), that light may either be reflected within the system, or it may be backscattered and ultimately enter the glass windows (clad rods) 220 and 222 and be propagated through the windows 220 and 222 and onto the dichroic filters 111 and 113 coated onto the surface of each photodetector 110 and 112, and ultimately passed into the photodiode where the energy from that backscattered or reflected photon is also converted to current that is then indistinguishable from the analyte modulated signal. Inside the existing sensor 100, because of backscatter, natural edge emission from the light source 108, simple reflections from materials within the device 100, and the principle of total internal reflection within the encasement, excitation light 129 can propagate through the waveguide at all angles. FIG. 2 illustrates the variety of angles at which the reflected excitation light 129 may enter the glass windows (clad rods) 220 and 222 before being propagated onto the dichroic filters 111 and 113.
The performance of the sensor 100 may be degraded when light other than the analyte modulated light, which is emitted by excited indicator molecules in the graft 106, enters the first/signal channel photodetector 110 and is measured thereby. Similarly, in sensors 100 having a reference channel, the performance of the device may be degraded when light other than the reference light enters the second/reference channel photodetector 112 and is measured thereby. That is, unwanted light is noise which may compromise the performance, accuracy, and/or sensitivity of the sensor 100.
Because of the physical limitation of incident light angle sensitivity of dichroic film type filters, unwanted backscatter or reflected excitation light 129 at high angles of incidence in the signal channel may be a significant source of noise, drift, and elevated baseline in the existing sensor 100. Accordingly, there is a need for sensors having improved accuracy and in which these problems are substantially reduced or eliminated.